1. Technical Field
The present invention relates in general to improved slider processing and, in particular, to an improved system, method, and apparatus for integrating lapping, air bearing patterning, and debonding in a releasable slider processing fixture.
2. Description of the Related Art
Current hard disk drive technology is continually being pushed to higher levels of density and speed concomitant with downward pressures on cost. The ability to efficiently design, evaluate, improve, and subsequently fabricate pole tip geometry, air-bearing sliders, and suspensions is deemed critical for success in keeping up with these demands. The main contributor to manufacturing cost is the lapping process with its associated need for handling of individual sliders to (a) place sliders in the lapping fixture, and to (b) transfer the rows or individual sliders from the lapping fixture to a carrier for subsequent photo-patterning.
The current preference for the individual slider based process is due the limited accuracy and the limited yield of the row based lapping process. A two-step process is used mainly because of capacity limitations of the lapping actuator (typically, less than 60 sliders—or one row—can be processed at once), and because the two-step process allows elimination of bad sliders before the photo-process. Currently, air bearing patterning of the air-bearing surface of read-write heads is a multi-step photo-patterning process with limited yield and chronic reliability problems. The main contributors to the problems are the bonding process (and associated debonding process) and the planarization material.
Thin film inductive heads and resistive read heads require a very small, constant effective magnetic spacing between their pole tips and the magnetic medium for high density operations. In addition, the pole tips of both heads must be trimmed to the exact length and lateral dimension. In a hard disk drive, this spacing must be kept constant while the head travels with respect to the rotating disk at a relative speed of several meters per second.
Past predictions of the demise of hard disk drive technology due to an anticipated limit in areal density of magnetic recording have been proven wrong because the effective magnetic spacing has been reduced beyond expectations. In addition to improved lithographies, both thin film and lubrication technologies are two main factors that allowed the observed progress. In the future, air bearing processing technology and accurate lapping while fully monitoring the electrical properties during the lapping process, will be crucial for further progress. However, the overall task is becoming more challenging, mainly because of excessive pressure to reduce the cost associated with this highly accurate processing.
Air bearings are the most efficient means to guarantee constant spacing, even at variable operation modes caused by the head descending from the headrest, during large external acceleration, temperature, humidity, and pressure changes. An air bearing is a carefully designed airfoil on the disk facing side of the physical structure carrying both the read and write head. The disk-facing side of this structure called slider must be patterned with high accuracy so that the airfoil meets the required specifications.
Slider lapping is needed to trim the read sensor to its final shape such that the resistance of the giant magneto-resistance element is within specifications. At the same time, the pole tip of the write head must be shortened and trimmed to its optimal size. Depending on the choice of the lapping conditions, the read-and-write pole tips are either directly coplanar with the ceramic surface or slightly recessed. This job can only be performed optimally when the endpoint of the lapping can be detected electrically and the process stopped after having reached the specified value.
Lapping is currently performed on a row of sliders containing approximately 40 to 60 sliders while only detecting the endpoint of a subset of reference sliders. Due to the warping and twisting of the row, the yield of this process is very low. Competing approaches now use shorter rows of sliders containing only about six sliders where all sliders are monitored. To allow mechanical addressing of individual sliders, rows are pre-parted such that they can be bent and twisted more easily. This allows an actuation of individual sliders with only minor cross talk to neighboring sliders. However, this process is not optimal because of the cross talk among sliders and because the small numbers of sliders that can be processed at once compared to the 40+ slider rows.
A final improvement of the pending accuracy problems could be achieved through a movement to individual slider lapping. To make this process economically more viable than current processes, more than six sliders or preferably even more than 40 sliders should be processed at once on one lapping machine. This requires a complex fixture with multiple grippers, connectors, and actuators. Miniaturized actuators for slider lapping use electromagnetic principles and are typically spaced at about one inch. Other types of actuators could be built using piezo-actuation or thermal actuation, but the fixtures would be expensive and difficult to miniaturize at the levels required for dense processing of pico and femto-format sliders with a pitch of less than 1.5 mm.
Prior to lapping, the putative ABS surface is polished to a high quality and lapping is only used to improve the surface quality while optimizing the pole tip geometry. The lapped surface must be exactly co-planar with the existing polished surface. Because gripping of small pieces like sliders cannot be done with such co-planarity, the lapping fixture requires a freedom to tilt in two directions of space according to the existing reference surface, which is a process known as gimbaling. Lapping of several sliders at once requires individual gimbaling in addition to the end point detection and actuation mentioned above.
One of the major problems of the current air bearing patterning process is the removal and cleaning of the sliders (both individual sliders and slider rows) from the carrier. During the bonding process, the sliders or rows are fixed to the carrier using a UV-curable acrylate. The solvent resistance against typical solvents needed during photo-patterning steps dominates the choice of this material. The same boundary condition applies to the planarization material. This material is also required to survive the photo-patterning steps for the formation of the air-bearing surface (ABS) of the sliders.
However, the solvent resistance is a major obstacle during the removal or debonding phase. Strongly-cured polymer networks are insoluble in all commercially accessible solvents and only can be removed by a process that combines strong swelling of the polymer with mechanical abrasion. Typical solvents used for this swelling-based removal are hot N-methyl pyrrolidone (NMP) or hot PGMEA. Mechanical abrasion is induced by a jet of soda particles (soda blast) or by rotating brush (brush cleaning). The disadvantage of this approach is the creation of sticky particles because swelling renders a polymer sticky and abrasion can tear particles off the matrix. These particles will stick to the active slider surfaces and have to be removed in a cumbersome process that involves a lot of solvent and extended cleaning times.
Use of silicone rubber for replication of complex structures is very widespread in dentistry and art. The process of creating a replica starts with the application of a separating medium to the surface of the article to be replicated (an ultra-thin layer of oil or soap). A room temperature cure or thermal cure silicone elastomer material is then applied such that it covers the entire surface as a thick film. Alternatively, an additional vessel is added around the object such that the void can be filled. The silicone material is then allowed to cure. Separation even of partially-trapped features is possible because the elastic nature of silicone rubber allows large but fully reversible dimensional changes (by a factor of two in many cases).
Currently, the best way to apply separation layers to technological objects made of glass, silicon, or ceramic is the exposure of parts of the surfaces to a vapor of fluorinated trichlorosilanes. Separation from these surfaces is possible without leaving behind macroscopic amounts of materials. At most, a monolayer of silicon oil may be left on the replicated surface. Silicone oils can be removed easily by dissolution in mildly polar solvents.
Swelling of rubbers is inversely proportional to the Young's modulus and depends on the similarity between the polymer network and the solvent (i.e., the gain in entropy when the two systems mix). In the case of poly dimethylsiloxane (PDMS), polar hydrophilic solvents like water or ethanol have a very small tendency to swell PDMS. This tendency can be reduced by using a mechanically stiff PDMS and by kinetically slowing the solvent uptake by means of a diffusion barrier. The simplest diffusion barrier is a thin layer of plasma polymerized PDMS created by a short exposure in a plasma asher.
Creation of electrical contacts through PDMS or along its surface is challenging because the elastic rubber is not compatible with more stable metallic conductors. Two approaches exist that can reliably form and maintain a contact even under slightly variable geometric conditions: (i) surface metallization, and (ii) electrical contacts through PDMS.
Surface metallization, e.g., covering PDMS with metal layers, can create electrical links that maintain contact during mechanical changes. Electrical contacts through PDMS are described in U.S. Pat. Nos. 5,371,654 and 5,635,846. These patents describe a PDMS layer with arrays of tilted or meandering wires. These designs form reliable electrical contacts between geometrically warped surfaces with no need for soldering. The structure is compliant in the vertical dimension and therefore allows multiple assembly and disassembly of chips onto surfaces.
Fabrication of the interconnection package starts from a metallic base onto which an array of wires is bonded and cut at a desired length, preferably tilted with 5 to 60 degrees from the vertical. A ball can be formed on the end of the wire, which is not bonded to the surface, using a laser or an electrical discharge. After the wire bonding process is completed, the substrate is placed in a casting mold and filled with a metered amount of PDMS. The substrate is extracted from the mold after the elastomer is cured. Similar systems are described in U.S. Pat. Nos. 5,947,750 and 6,133,072.
Recent variants of PDMS are mechanically well understood such that materials can be designed to match mechanical requirements. Deformations can be described with a mathematical model using equations of elastic theory to exactly describe this effect. See, e.g., Bietsch and Michel, “Conformal contact and pattern stability of stamps used for soft lithography”, J. Appl. Phys. 88, 4310 (2000); Johnson, “Contact Mechanics”, Cambridge University Press, Cambridge (1985); and S. P. Timoshenko and J. N. Goodier, “Theory of Elasticity”, Mc-Graw-Hill, New York). The mechanics of flat PDMS posts can be controlled such that their geometry changes to allow the front surface to have full areal contact with the substrate—a process known as gimbaling.
PDMS-actuated valves have been described in the context of biosensors where pressurized air can be used to switch a liquid flow. This is done by inflating a cavity in the vicinity of the liquid channel. The increase of pressure in the cavity leads to the collapse of the liquid channel and thus pinches off the flow. This process is reversible and allows many actuation cycles. This type of actuation is not very precise dimensionally, but the control over the force via the air pressure is acceptable.